Most toroidal continuous variable transmission have a toroidal rotary speed changer that is composed of an input disk driven by an input shaft, an output disk arranged confronting with the input disk and connected to an output shaft, and adjustable power rollers arranged in frictional rolling-contact with both the disks. In the toroidal continuous variable transmission, varying the tilt of the power roller causes the continuous variable variation in the speed of rotation that is to be transmitted from the input disk to the output disk.
An example of toroidal continuous variable transmissions conventionally installed on vehicle is diagrammatically shown in FIG. 6. A toroidal continuous variable transmission 1 installed on the vehicle is comprised of an input shaft 21 for taking off the power of an engine E, an input disk 3 supported for rotation relatively to the input shaft 21, an output disk 23 connected to an output shaft 24 and arranged in opposition to the input disk 3 while for rotation relatively to the input disk 3, a pair of power rollers 2, 2 adjustable in tilt and arranged between the confronting input disk 3 and the output disk 23 so as to transmit the applied torque from the input disk 3 to the output disk 23, and thrust means 22 such as a loading cam provided between a flange 25 attached to the input shaft 21 and the input disk 3 so as to control the magnitude of the contact pressure of the input disk 3 against the output disk 23 dependently on the magnitude of the applied torque, whereby adjusting the tilt of the power rollers 2, 2 may result in the continuous variable speed changing of rotation that is transmitted from the input shaft 3 to the output shaft 32. If the power rollers 2, 2 were adjusted at a tilt angle .theta. shown in the drawings, the power rollers 2, 2 would be in friction contact with the input disk 3 at a radius r.sub.1 while in friction contact with the output disk 23 at a radius r.sub.2 and, therefore, the output speed would be r.sub.1 /r.sub.2 the input speed. In the meantime the members at 4 are trunnions for supporting the adjustable power rollers 2, 2 and will be described in detail later. The tilt angles of the power rollers 2, 2 in the toroidal continuous variable transmission 1 are adjusted by means of a controller unit that will be described hereinafter.
Japanese Patent Laid-Open No. 151219/1995 discloses the toroidal continuous variable transmission of the type described just above and the speed-change type described just above and the speed-change control system therefor. FIG. 7 shows an exemplary prior art of the toroidal continuous variable transmission and the speed-change control system therefor. As apparent from the illustration, a pair of adjustable power rollers 2, 2 is arranged confronting with each other between the input and output disks 3, 23. The adjustable power rollers 2, 2 are each supported for pivoting or rocking motion in response to deflections of the space between the confronting input and output disks 3, 23 along the axial direction of the disks, while for rotation in frictional rolling-contact with the input and output disks 3, 23. This support mechanism for the power rollers 2, 2 is a kind of pivot assembly composed of a supporting member, what is called trunnion 4, and a supporting shaft 5 serving as a pivot shaft for the trunnion 4. The trunnions 4, 4 are each mounted to a transmission case (not shown) for rotation as well as axial movement. Moreover the trunnions 4, 4 each have a pivotal shaft, or pivotal shaft 6, and are movable along the axial direction of the pivotal shaft 6 while capable of pivoting movement on the pivotal shaft 6. Connected to the pivotal shaft 6 of the trunnion 4 is a piston 7 that is fitted for sliding movement in a hydraulic cylinder 8 formed in the transmission case. The hydraulic cylinder 8 is divided into two cylinder chambers, or an acceleration chamber 8a and deceleration chamber 8b, with the piston 7.
The cylinder chambers 8a, 8b are each communicated with a sliding-spool valve 10 through hydraulic conduits 9a, 9b. The sliding-spool valve 10 has therein a spool 11 that is movable in a sliding manner and kept at its neutral position by the preload of centering-springs arranged on the opposing ends of the spool 11, one on each end. The sliding-spool valve 10 is provided at opposing ends thereof with ports Sa, Sb, one on each end, the port Sa of which is applied with a hydraulic pressure Pa through a solenoid-actuated valve 13a while the port Sb is open to a solenoid-actuated valve 13b. The sliding-spool valve 10 is further provided with ports PL, A, B and R, the port PL being connected to a hydraulic pressure source, the port A being connected to the acceleration chamber 8a through the hydraulic conduit 9a, the ports B being connected to the deceleration chamber 8b through the hydraulic chamber 9b, and the two ports R being open to oil reservoir. The solenoid-actuated valves 13a, 13b may be actuated in response to control signals from a controller unit 14. It will be thus understood that the solenoid-actuated valves 13a, 13b function as valves for controlling the speed ratio of the output speed to the input speed in the toroidal continuous variable transmission.
Any one of the pivotal shafts 6, 6 is provided at its end with a precessional cam 15, against which is abutted one extremity of a lever 16 that is connected at its opposite extremity to a potentiometer 17. The precessional cam 15 may be made to move in proportion to the axial-linear displacement Y as well as the angular displacement .theta. of its associated pivotal shaft 6 of the trunnion 4. If both of the linear and angular displacements Y, .theta. take place on the pivotal shaft 4, the potentiometer 17 will detect the resultant displacement of the linear and angular displacements and produce in proportion to the resultant displacement a potential output V, which is in turn applied to the controller unit 14. The integrated mechanism of the precessional cam 15, lever 16 and potentiometer 17 functions as the detecting means that may detect the potential proportional to the resultant displacement and apply a correction signal to the controller unit 14, which in turn controls the speed-ratio control valve so as to make the speed ratio of the output speed to the input speed coincide with a desired speed ratio. The controller unit 14 is moreover incorporated with a tachometer 18 at a power-take-off shaft, an engine tachometer 19 and accelerometer 20. The controller unit 14 may be applied with signals regarding the speed-changing information such as the rotational frequency of the power take-off shaft, the rotational frequency of the engine, the depression of the accelerator pedal or the like. As an alternative, a vehicle speed sensor may be used in place of the power take-off tachometer 18 and also a throttling sensor may be employed in place of the accelerometer 20.
In the toroidal continuous variable transmission as described just above, when the trunnion 4 is displaced towards any one direction along the pivotal shaft (or the axial direction of the pivotal shaft 6), the power roller 2 may move to thereby shift the rolling-contact circles of the input and output disks 3, 23 with the power roller 2. Owing to the characteristic in which the power roller 2 may make the pivoting movement on the pivotal shaft 6 in the direction and at the speed that occur dependent on the direction and amount of the displacement along the pivotal shaft 6 of the trunnions 4, the continuous variable speed changing may be achieved by adjusting the angular deflection of the pivotal shaft of the trunnion 4.
For transmitting the rotation of the input shaft 21 to the output shaft 24 with changing the ratio of the speed of the input shaft 21 to the speed of the output shaft 24, the loading cam 22 is provided to help ensure the large contact pressure at each rolling-contact area between the power roller 2 and any one of the input and output disks 3, 23. Consequently, regardless of wherever the axial reference position may be in the toroidal continuous variable transmission 1, the input and output disks 3, 23 and power rollers 2, 2, as moving away from the axial reference position due to the tolerance as well as deflection, may be forced to shift dependent on the axial thrust force caused by the cam action of the loading cam 22. On the toroidal continuous variable transmission of double-cavity type in which two set of toroidal speed-changing units are arranged along the axial direction of the power transmitting line of the input shaft 21 and output shaft 24, especially, the power rollers 2, 2 in at least any one of the toroidal speed-changing units are apt to move away from the axial reference position, to thereby make the axial deflection larger. To cope with such axial deflection, the power rollers 2, 2 are supported on the trunnions 4, 4 for rotating as well as for pivoting movements so that the axial deflection occurring in the power rollers 2, 2 along the power transmitting line may be compensated with the pivoting movement of the power rollers 2, 2 on the supporting shafts 5, 5. Alternatively, if the trunnions 4, 4 occupied the axial reference position in the toroidal continuous variable transmission 1, there would be no need for supporting the power rollers 2, 2 on the trunnions 4, 4 for pivoting movement.
The mounting structure of the power rollers 2, 2 on the trunnions 4, 4 will be described with reference to FIGS. 8 and 9. Each power roller 2 is comprised of a rolling body 30, a back plate 31 and a thrust bearing 32 that may bear the rolling body 30 against the back plate 31 for rotation under the thrusting force, which is a force acting on the rolling body 30 along the rotating axis of the rolling body 30. The supporting shaft 5 is further to mount the power roller 2 to the trunnion 4. The supporting shaft 5 comprises a first journal section 36 and a second journal section 37 integral with the first journal section 36, the first journal section 34 being fitted in the trunnion 4 for rotation through a first journal bearing 34 of needle bearing type while the second journal section 37 supporting thereon the rolling body 30 through a second journal bearing 35 of needle bearing type. Alternatively, both the first and second journal sections 36, 37 may be formed separately from each other and connected integrally with each other. As shown in FIG. 8, the first and second journal sections 36, 37 of the power roller 2 are eccentric with each other. The rolling body 30 and back plate 31 are fitted on the second journal section 37 so as to keep the concentricity with each other. Moreover the back plate 31 is born against the trunnion 4 through a thrust bearing 33 of a sliding bearing, needle bearing or the like, which is to bear an urging force exerted from the input and output disks 3, 23. The thrust bearing 32 has rolling elements of ball, which are contained in a cage 38 and held between confronting design raceway grooves that are provided on the opposing roller body 30 and back plate 31.
For miniature structure of the power roller 2 in FIG. 8, the needle bearings are preferably used for the first journal bearing 34 to support the first journal section 36 of the supporting shaft 5 in the trunnion 4 and the second journal bearing 35 to support the rolling body 30 on the second journal section 37 of the supporting shaft 5. The needle bearings 34, 35 each, principally, have a radial clearance while the needles are subject to the crowning at their opposing ends that would be otherwise exposed to the concentrated load. Nevertheless, this causes a major problem in which a lean is liable to arise between the trunnion 4 and the supporting shaft 5 while between the rolling body 30 and the supporting shaft 5.
In addition, the fitting of the back plate 31 on the second journal section 37 of the supporting shaft 5 is carried out by loose fit process in order to make possible the easy assembly of the power roller 2. And then, the back plate 31 is too thin in thickness to resist to a lean of the supporting shaft 5. Therefore, the axis line C--C of rotation of the rolling body 30 in the power roller 2 is apt to sometimes fluctuate due to disturbances or the like even if the desired speed ratio of the toroidal continuous variable transmission is always constant.
For example, as in the case where the torque to be transmitted through the toroidal continuous variable transmission 1 underwent a change in magnitude under such situation that the supporting shaft 5 can not be kept away from its lean, if the rolling body 30 of the power roller 2 undergoes a change in the direction of the tangential force applied to the rolling body 30 from the input and output desks 3, 23, the axis line C--C of rotation of the rolling body 30 in the power roller 2 moves along the pivotal shaft 6. Where the axis line of rotation of the rolling body 30 in the power roller 2 exactly intersects with that of the input and output disks 3, 23, there happens no force to make the power roller 2 tilt or pivot. In contrast, when the axis line C--C of rotation of the rolling body 30 in the power roller 2 moves along the pivotal shaft 6 due to the lean of the supporting shaft 5 with respect to the trunnion 4 so as to deviate from the rotational axis of the input and output disks 3, 23, the power roller 2 is subject to the pivoting force that has a magnitude and acting direction in accordance with the deviation. FIG. 9 illustrates a power roller 2 that has deviated along the pivotal shaft 6 (towards upper side in the drawings)so that the axis of rotation of the rolling body 30 has deviated from its position C--C to another position C'--C'. Such deviation of the rotational axis C--C of the rolling body 30 causes simultaneously the displacement of the rolling-contact area of the rolling body 30 with both the input and output disks 3, 23 along the pivotal shaft 6 in the same direction with that of the deviation. This starts the speed changing in the toroidal continuous variable transmission 1, likewise with the principal of the speed governing in the transmission having toroidal surfaces. That is, this prior structure has a major problem in which the lean of the supporting shaft 5 may force the toroidal continuous variable transmission 1 to start the speed changing operation even if the desired speed ratio is fixed while the trunnion 4 is kept in its tilt.
Moreover the force exerted on the power roller 2 from the input and output disks 3, 23 involves a thrust force along the rotational axis of the power roller 2 and a reacting torque (tangential force) taking place at the transmission of torque. In addition, the reacting torque or tangential force may occur reversely in direction either under the driving condition where the applied torque is transmitted positively or under the coasting condition where the vehicle is carried by inertia. On a tilt of the supporting shaft 6 with respect to the trunnion 4, consequently, the deviation of the rotational axis C--C of the power roller 2 is different in direction, depending on either of the driving and the coast, so that the speed ratio may differ actually under even if the desired speed ratio is to be kept at constant.
On the driving under light load, the tangential force described just above is too small in magnitude to make stable the position of the rotational axis of the power roller 2 relatively to the trunnion 4. Further, as in the case the transmission was changed over between the driving position and the coast position, when the transmission undergoes a change of the torque large in amplitude, this causes an disadvantage in which the vibratory variations of the speed ratio may happen on the same desired speed ratio, resulting in making uncomfortable the riding comfort for the driver. Moreover the toroidal continuous variable transmission of double-cavity type has a major problem in which as the transient difference may occur in speed ratio between the two set of the toroidal speed-changing units, the speed ratio undergoes changes of relatively short cycle and/or the great torque is caused by the torque circulation in the speed-changing units with resulting in the possibility of transmission failures such as slippage in traction drive units.
A primary object of the present invention is to overcome the major problems as described above and more particular to provide a toroidal continuous variable transmission in which a back plate holds a supporting shaft so as to resist against a lean of the supporting shaft to thereby limit a deviation of the rotational axis of a rolling body in a power roller within a radial clearance in a needle bearing of the supporting shaft whereby the desired speed ratio may be kept steady regardless of the speed-governing operations of the toroidal continuous variable transmission.
Another object of the present invention is to provide a toroidal continuous variable transmission in which the type of bearings is selected so as to make wider the load-carrying surface area along the axial direction to thereby suppress a lean of the supporting shaft, resulting in limiting a deviation of the rotational axis of a rolling body in a power roller within a radial clearance in a needle bearing of the supporting shaft whereby the desired speed ratio may be kept steady regardless of the speed-governing operations of the toroidal continuous variable transmission.
In another aspect of the present invention, a toroidal continuous variable transmission is provided wherein the fitting structure of a supporting shaft into a back plate is designed so that a lean of the supporting shaft is suppressed so as to limit a deviation of the output disks 3, 23, the power roller 2 is subject to the pivoting force that has a magnitude and acting direction in accordance with the deviation. FIG. 9 illustrates a power roller 2 that has deviated along operations of the toroidal continuous variable transmission.